Vertical Structure in Atmospheric Fog and Haze and Its Effects on Visible and Infrared Extinction

Abstract

Vertical structure of the size distribution and number concentration of particulates in atmospheric fog and haze near Grafenwöhr, West Germany, were measured with a balloonborne light-scattering aerosol counter for periods spanning parts of eight days in February 1976. For haze (?5 km visibility) conditions, little vertical variation is seen; but for low visibility (<1 km) fog conditions, significant vertical increases in concentration of droplets with radii larger than 4 ?m are seen over the first 150 m altitude. For haze, the particle size distribution is approximated by a log-normal with geometric mean radius rg≈0.2 ?m and geometric standard deviation σg≈1.9. For fog, a bimodal distribution is found with a relative maximum for the larger particle mode at radii of 4 to 6 ?m and corresponding values rg≈5 ?m and σg≈1.6; the smaller particle mode has values of rg≈0.3 ?m to rg≈0.6 ?m and σg≈1.8 to σg≈2.5. Liquid water content values for haze and fog range from 10?4 to 0.45 g m?3. Extinction calculated from the particle size distributions shows an approximate 1/? wavelength dependence for haze conditions, but nearly neutral (wavelength independent) extinction for heavy fog. A correlation exists between calculated particulate extinction and calculated liquid water content, independent of particle size distribution, for the fogs and hues studied.